Driving higher voltages with p-type MOSFET

Hi guys:

I'd like to control a 24Vdc load by sourcing with a p-type power MOSFET. I'm driving the gate directly from a mictrocontroller that uses +5 and ground.. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.

Is there a simple, standard way that this is done? Thanks for any replies.

Don
Kansas City

 

[Grant]

On Mon, 20 Aug 2012 19:16:13 -0700 (PDT), eromlignod1@att.net wrote:

Hi guys:

I'd like to control a 24Vdc load by sourcing with a p-type power MOSFET. I'm driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.

Is there a simple, standard way that this is done? Thanks for any replies.

Safest is an optocoupler, you can get opto gate drivers; will
need a zener regulated supply hanging down from +24V.

If the switching speed allows, you could you a blocking capacitor
with a reset diode; no good for switched DC operation; beware the
power on charging of the blocking cap.

You could use an NPN transistor with a resistor up to the gate,
and a zener to limit gate voltage; fairly safe?

Lots of options, depending on desired switching rate and power.

Grant.

Don
Kansas City

 

[fungus]

On Tuesday, August 21, 2012 4:16:13 AM UTC+2, Don Gilmore wrote:

I'd like to control a 24Vdc load by sourcing with a p-type power MOSFET. I'm driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.

Is there a simple, standard way that this is done? Thanks for any replies.

Use a second transistor to control the voltage
to the MOSFET?

 

[Don Gilmore]

Well, it's actually half of a PNP/NPN pair that is switching back and forth with a common gate at about 20 kHz. The NPN is no problem: I can switch it with +5. It's the PNP that's vexing me. The opto idea sounds good, if I can find one that can handle the current (20 amps) and has a low drain-to-source resistance. It also needs to be surface mount and the board is very tiny.

Thanks for the ideas so far!

Don

 

[Don Gilmore]

On Tuesday, August 21, 2012 5:21:29 AM UTC-5, fungus wrote:
Use a second transistor to control the voltage to the MOSFET?

Yeah, but I need it to turn on the output on a "low" signal, so I'd still have to drive it with a PNP at +24 and +19, which presents the same problem.

It's part of a PNP/NPN pair and one circuit has to shut off when the other turns on with a common gate signal from the MCU. I could use two NPN's and invert one gate signal, but there are three of these pairs and I'm trying to keep my chip count down.

Don

 

[Tim Wescott]

On Tue, 21 Aug 2012 05:50:39 -0700, Don Gilmore wrote:

Well, it's actually half of a PNP/NPN pair that is switching back and
forth with a common gate at about 20 kHz. The NPN is no problem: I can
switch it with +5. It's the PNP that's vexing me. The opto idea sounds
good, if I can find one that can handle the current (20 amps) and has a
low drain-to-source resistance. It also needs to be surface mount and
the board is very tiny.

Thanks for the ideas so far!

Don

I'd use all N-channel parts and a half-bridge driver like the LM5109 or
any of the plethora of other parts out there.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

 

[George Herold]

On Aug 21, 9:08 am, Don Gilmore <eromlign...@att.net> wrote:

On Tuesday, August 21, 2012 5:21:29 AM UTC-5, fungus wrote:

Use a second transistor to control the voltage to the MOSFET?

A little resistor divider chain and npn at the bottom to switch it,
sounds easy.

Yeah, but I need it to turn on the output on a "low" signal, so I'd still have to drive it with a PNP at +24 and +19, which presents the same problem.

Add an inverter between the micro and the npn as switch. Or code the
micro differently? ... or yet another transistor?

George H.

It's part of a PNP/NPN pair and one circuit has to shut off when the other turns on with a common gate signal from the MCU.  I could use two NPN's and invert one gate signal, but there are three of these pairs and I'm trying to keep my chip count down.

Don

 

[fungus]

On Tuesday, August 21, 2012 3:08:19 PM UTC+2, Don Gilmore wrote:

It's part of a PNP/NPN pair and one circuit
has to shut off when the other turns on with
a common gate signal from the MCU.

Use a resistor inverter:

24V
|
R1
|
+---to MOSFET gate
|
BJT --R2--MCU
|
|
GND

When the MCU switches the BJT "on" the
gate of the MOSFET will go to ground.

When the MCU switches the BJT "off" the
gate of the MOSFET will go up to 24V.

It seems too simple, I must be missing
something...

 

[John Fields]

On Mon, 20 Aug 2012 19:16:13 -0700 (PDT), eromlignod1@att.net wrote:

Hi guys:

I'd like to control a 24Vdc load by sourcing with a p-type power MOSFET. I'm driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.

Is there a simple, standard way that this is done? Thanks for any replies.

Don
Kansas City

I think this is the simplest implementation:

PNP
24VDC>-----+--E B C-+
| | |
[R1] | |
| | |
+----+ |
| |
[3.9R1] [LOAD]
| |
C |
MCU>-[R2]--B NPN |
E |
| |
GND--------+--------+

--
JF

 

[John Fields]

On Tue, 21 Aug 2012 08:08:05 -0700 (PDT), fungus <tooby@artlum.com>
wrote:

On Tuesday, August 21, 2012 3:08:19 PM UTC+2, Don Gilmore wrote:

It's part of a PNP/NPN pair and one circuit
has to shut off when the other turns on with
a common gate signal from the MCU.


Use a resistor inverter:

24V
|
R1
|
+---to MOSFET gate
|
BJT --R2--MCU
|
|
GND

When the MCU switches the BJT "on" the
gate of the MOSFET will go to ground.

When the MCU switches the BJT "off" the
gate of the MOSFET will go up to 24V.

It seems too simple, I must be missing
something...

---
Yup, you punched through the MOSFET's gate oxide.

--
JF

 

[John Fields]

On Tue, 21 Aug 2012 16:50:04 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Mon, 20 Aug 2012 19:16:13 -0700 (PDT), eromlignod1@att.net wrote:

Hi guys:

I'd like to control a 24Vdc load by sourcing with a p-type power MOSFET. I'm driving the gate directly from a mictrocontroller that uses +5 and ground. The problem is that I need +24 and +19. I looked into "high voltage level translators", but they have been discontinued or are a non-stock item.

Is there a simple, standard way that this is done? Thanks for any replies.

Don
Kansas City

I think this is the simplest implementation:

PNP
24VDC>-----+--E B C-+
| | |
[R1] | |
| | |
+----+ |
| |
[3.9R1] [LOAD]
| |
C |
MCU>-[R2]--B NPN |
E |
| |
GND--------+--------+

---
Oops, PNP should read "PMOS".


--
JF

 

[whit3rd]

On Monday, August 20, 2012 7:16:13 PM UTC-7, Don Gilmore wrote:

I'd like to control a 24Vdc load by sourcing with a p-type power MOSFET. I'm driving the gate directly from a mictrocontroller that uses +5 and ground.

Why do you want to use a PMOS? If you want to switch a 24V circuit ON with 5V logic low,
the PMOS is going to be inconvenient (the gate and source which control its
conduction are up at the +24V side of the circuit).

I'll assume that you want the load to always have one pin grounded, and want to
do a high-side switch between +24V and its other pin; that's a good reason to
switch with a PMOS device.

So, PMOS high-side switch, with source connected to +24, gate pullup resistor
to source, and gate pulldown with an NPN transistor, base connected to +5V, emitter
through resistor to your logic pin. You can put a resistor from NPN collector to the
gate, instead of straight connection, if the PMOS gate doesn't tolerate 19V.

Optoisolators are expensive and low-performance by comparison with even the lowliest
simple transistor.

 

[John Fields]

On Wed, 22 Aug 2012 02:47:36 -0700 (PDT), fungus <tooby@artlum.com>
wrote:

On Tuesday, August 21, 2012 11:55:36 PM UTC+2, John Fields wrote:

It seems too simple, I must be missing
something...

---

Yup, you punched through the MOSFET's gate oxide.


Oh, yes... :-(

Ok, add an extra resistor in parallel with
the BJT.

24V
|
R1
|
+---to MOSFET gate
|
/-+-\
| |
R3 |
| |
| BJT --R2--MCU
| |
\ /
GND

Now when the BJT is closed the voltage
at the MOSFET gate is proportional to
the values of R1 and R3 (hopefully low
enough to keep the magic smoke inside
the MOSFET).

---
No.

When the BJT is open the MOSFET's gate will see

(24V * R3) / (R1 + R3)

but when the BJT is conducting, the MOSFET's gate will be sitting at
about 0.3V above ground.

That means that the drain-to-gate voltage will be about 23.7V, which
is higher than the usual +/- 20V spec.

The right way to do it, I think, is like this:

.. PMOS
..24VDC>-----+--D G S-+
.. | | |
.. [R1] | |
.. | | |
.. +----+ |
.. | |
.. [3.9R1] [LOAD]
.. | |
.. C |
..MCU>-[R2]--B NPN |
.. E |
.. | |
..GND--------+--------+

That way, when the input from the MPU is low the NPN won't be
conducting, the PMOS gate will be at 24V and so will the drain, so the
MOSFET will be off.

Then, when the MCU output goes high the NPN will turn on, connecting
3.9R1 to ground.

That'll pull the gate voltage down to about 19V, 5 volts lower than
the drain voltage, which is what Don asked for.

--
JF

 

[fungus]

On Tuesday, August 21, 2012 11:55:36 PM UTC+2, John Fields wrote:

It seems too simple, I must be missing
something...

---

Yup, you punched through the MOSFET's gate oxide.

Oh, yes... :-(

Ok, add an extra resistor in parallel with
the BJT.

24V
|
R1
|
+---to MOSFET gate
|
/-+-\
| |
R3 |
| |
| BJT --R2--MCU
| |
\ /
GND

Now when the BJT is closed the voltage
at the MOSFET gate is proportional to
the values of R1 and R3 (hopefully low
enough to keep the magic smoke inside
the MOSFET).

 

[John Fields]

On Wed, 22 Aug 2012 06:41:03 -0500, John Fields
<jfields@austininstruments.com> wrote:

The right way to do it, I think, is like this:

. PMOS
.24VDC>-----+--D G S-+
. | | |
. [R1] | |
. | | |
. +----+ |
. | |
. [3.9R1] [LOAD]
. | |
. C |
.MCU>-[R2]--B NPN |
. E |
. | |
.GND--------+--------+

---
Aarghhh...

.. PMOS
..24VDC>-----+--S G D-+
.. | | |
.. [R1] | |
.. | | |
.. +----+ |
.. | |
.. [3.9R1] [LOAD]
.. | |
.. C |
..MCU>-[R2]--B NPN |
.. E |
.. | |
..GND--------+--------+

That way, when the input from the MPU is low, the NPN won't be
conducting, the PMOS gate will be at 24V and so will its source, so
the MOSFET will be off.

Then, when the MCU output goes high the NPN will turn on, connecting
3.9R1 to ground.

That'll pull the gate voltage down to about 19V, 5 volts lower than
the source voltage, which is what Don asked for.

--
JF

 

[Don Gilmore]

On Wednesday, August 22, 2012 7:05:42 AM UTC-5, John Fields wrote:

--B NPN | . E | . | | .GND--------+--------+ That way, when the input from the MPU is low, the NPN won't be conducting, the PMOS gate will be at 24V and so will its source, so the MOSFET will be off. Then, when the MCU output goes high the NPN will turn on, connecting 3.9R1 to ground. That'll pull the gate voltage down to about 19V, 5 volts lower than the source voltage, which is what Don asked for. -- JF

Yes, this is what I was looking for. Thanks John.

Don

 

[John Fields]

On Wed, 22 Aug 2012 05:27:19 -0700 (PDT), Don Gilmore
<eromlignod1@att.net> wrote:

Yes, this is what I was looking for. Thanks John.

Don

---
My pleasure.

--
JF

 

[Don Gilmore]

On Wednesday, August 22, 2012 1:55:26 PM UTC-5, John Fields wrote:

On Wed, 22 Aug 2012 05:27:19 -0700 (PDT), Don Gilmore wrote: >Yes, this is what I was looking for. Thanks John. > >Don --- My pleasure. -- JF

Well, wait. I guess this works, but it still doesn't do what I want.

This switches on the load when the output is HIGH. If I wanted to that, I could have just used an n-type MOSFET and controlled it directly with the MCU.

What I need is for the load to switch on when I give it a LOW. And as soon as you start thinking about using a PNP to switch it, you run into the same problem.

Am I stuck with an inverter?

Don

 

[Grant]

On Wed, 22 Aug 2012 17:17:33 -0700 (PDT), Don Gilmore <eromlignod1@att.net> wrote:

On Wednesday, August 22, 2012 1:55:26 PM UTC-5, John Fields wrote:
On Wed, 22 Aug 2012 05:27:19 -0700 (PDT), Don Gilmore wrote: >Yes, this is what I was looking for. Thanks John. > >Don --- My pleasure. -- JF

Well, wait. I guess this works, but it still doesn't do what I want.

This switches on the load when the output is HIGH. If I wanted to that, I could have just used an n-type MOSFET and controlled it directly with the MCU.

What I need is for the load to switch on when I give it a LOW. And as soon as you start thinking about using a PNP to switch it, you run into the same problem.

Am I stuck with an inverter?

+24 o--------------o---.-. .----
| | A |
| =====
| |
| | |
| | |
|_| |
| |
o-----'
|
| |
| |
|_|
|
|/
+Vdd o-----o-----|
| |>
| |
------- |
| | |
| CPU |o--'
| |
-------
|
0V o-------o---------

Now high on CPU output turns off the transistor, thus no gate voltage?

You might want a b-e resistor so the circuit is failsafe off while CPU
starts, before the outputs are enabled. Voltage divider to prevent
excess gate voltage as in other posts.

Grant.

Don

 

[Jasen Betts]

On 2012-08-23, Don Gilmore <eromlignod1@att.net> wrote:

On Wednesday, August 22, 2012 1:55:26 PM UTC-5, John Fields wrote:
On Wed, 22 Aug 2012 05:27:19 -0700 (PDT), Don Gilmore wrote: >Yes, this is what I was looking for. Thanks John. > >Don --- My pleasure. -- JF

Well, wait. I guess this works, but it still doesn't do what I want.

This switches on the load when the output is HIGH. If I wanted to
that, I could have just used an n-type MOSFET and controlled it
directly with the MCU.

What I need is for the load to switch on when I give it a LOW. And
as soon as you start thinking about using a PNP to switch it, you run
into the same problem.

Am I stuck with an inverter?

connect the NPN base resistor to the +5V supply and the NPN emitter to the
MCU output.

Don

--
⚂⚃ 100% natural

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